What Preheat Temperature Does A36 Need?
A36 preheat depends on your welding process and material thickness. D1.1:2025 Table 5.11 assigns A36 to Category A for non-low-hydrogen SMAW or Category B for GMAW, FCAW, SAW, and low-hydrogen SMAW, with minimums ranging from 32°F to 300°F.
The Short Answer
If you are welding A36 with GMAW, FCAW, SAW, or low-hydrogen SMAW electrodes (like E7018), your preheat comes from Category B in Table 5.11. For material 3/4 inch and under, the minimum is 32°F—no preheat above ambient in normal shop conditions.
If you are using non-low-hydrogen SMAW electrodes (like E6013 or E6010), A36 falls under Category A. The same 3/4-inch-and-under thickness still shows 32°F, but above 3/4 inch the numbers diverge sharply: 150°F for Category A versus 50°F for Category B at the same thickness.
That 100°F difference from electrode choice alone is the detail most preheat references miss.
Table 5.11 Values for A36
Category A — SMAW with Non-Low-Hydrogen Electrodes
| Thickness of Thickest Part | °F | °C |
|---|---|---|
| 1/8 to 3/4 in. incl. | 32* | 0* |
| Over 3/4 thru 1-1/2 in. incl. | 150 | 65 |
| Over 1-1/2 thru 2-1/2 in. incl. | 225 | 110 |
| Over 2-1/2 in. | 300 | 150 |
Category B — SMAW with Low-Hydrogen Electrodes, SAW, GMAW, FCAW
| Thickness of Thickest Part | °F | °C |
|---|---|---|
| 1/8 to 3/4 in. incl. | 32* | 0* |
| Over 3/4 thru 1-1/2 in. incl. | 50 | 10 |
| Over 1-1/2 thru 2-1/2 in. incl. | 150 | 65 |
| Over 2-1/2 in. | 225 | 110 |
*Footnote a (Table 5.11): When the base metal temperature is below 32°F, the base metal shall be preheated to a minimum of 70°F and the minimum interpass temperature shall be maintained during welding. This applies to both categories.
Why Two Categories for the Same Steel?
The category system in Table 5.11 is driven by hydrogen risk, not steel strength. A36 has a yield strength of 36 ksi and a tensile range of 58–80 ksi—moderate by structural steel standards. The steel itself is not particularly crack-sensitive. The variable is how much hydrogen your welding process introduces.
Non-low-hydrogen electrodes like E6013 deposit more diffusible hydrogen into the weld and heat-affected zone. Hydrogen migrates to areas of high stress concentration and, if enough accumulates before it can diffuse out, initiates cold cracking. Thicker material cools faster, trapping hydrogen before it escapes. That combination—higher hydrogen deposit plus faster cooling—is why Category A demands significantly more preheat above 3/4 inch.
Low-hydrogen electrodes (E7018, E7016, E7028) and gas-shielded processes (GMAW, FCAW) deposit substantially less hydrogen, which is why Table 5.11 groups them together in Category B with lower preheat requirements.
The Thickness Breakpoint Fabricators Miss
Picture this: your shop is welding 1-inch A36 plate with E6013 because you have pallets of it and the job is non-critical. Table 5.11 says 150°F preheat (Category A, over 3/4 thru 1-1/2 inch). Switching to E7018 drops that to 50°F (Category B, same thickness band). The electrode change saves you 100°F of preheat on every joint—less gas, less waiting, faster production. And the weld is stronger.
The key breakpoint is 3/4 inch. Below that thickness, both categories require only 32°F—ambient conditions in most shops. Above 3/4 inch, Category A jumps to 150°F while Category B only requires 50°F. That gap narrows at higher thicknesses (225°F vs. 150°F at 1-1/2 to 2-1/2 inch) but never closes until you get well above 2-1/2 inch.
Per Clause 5.7.1, these are minimum values. D1.1 does not set a maximum preheat for A36 because it is not a quenched-and-tempered steel. You can preheat higher if conditions warrant it—cold ambient temperature, high restraint, or thick multipass joints—without violating the code.
Mixed Joints: Which Preheat Governs?
Clause 5.7.2 is clear: when a joint connects base metals with different minimum preheats (based on category and thickness), use the highest minimum preheat. If you are welding 1-inch A36 (Category B, 50°F) to 1-inch A572 Grade 50 (also Category B, 50°F), the preheat is 50°F. But if the A572 is Grade 65 in Category C, the joint preheat follows Category C—which requires 150°F at that thickness.
The thickness of the thickest part at the point of welding determines which row of Table 5.11 applies. For a tee joint where a 3/8-inch A36 web meets a 1-inch A36 flange, you use the 1-inch flange thickness to look up the preheat, not the 3/8-inch web.
Footnote a — The Cold-Weather Rule
Every 32°F entry in Table 5.11 carries footnote a. If you are fabricating or erecting in winter and the steel temperature is below 32°F, you must preheat to at least 70°F and maintain that as your minimum interpass temperature. This is not optional. The 32°F entry does not mean “no preheat ever”—it means no preheat above ambient when ambient is above freezing.
Clause 7.11.2 adds a separate prohibition: welding shall not be done when the ambient temperature at the weld is lower than 0°F. And per Clause 7.6.2, the preheat zone must extend in all directions from the point of welding—at least twice the base metal thickness for material under 1-1/2 inch, or at least equal to the base metal thickness (but not less than 3 inches) for material 1-1/2 inch and thicker.
To confirm your steel is A36, check the mill test report (MTR) — it lists the ASTM specification, which maps to a Table 5.6 base metal group and then to Table 5.11 preheat category.
Frequently Asked Questions
For material 3/4 inch and under, Table 5.11 lists 32°F for both Category A and Category B. That means no preheat above ambient is required in normal conditions. However, footnote a states that if the base metal is below 32°F, you must preheat to a minimum of 70°F and maintain that as your interpass temperature.
Non-low-hydrogen electrodes like E6013 leave more hydrogen in the weld. Hydrogen trapped in the heat-affected zone causes cold cracking, especially in thicker material where the cooling rate is faster. Low-hydrogen electrodes like E7018, plus GMAW and FCAW processes, deposit less hydrogen, so D1.1 assigns them a lower preheat category.
D1.1 does not set a maximum preheat for A36 because it is not a quenched-and-tempered steel. Table 5.11 only sets minimums. Excessive preheat wastes time and money but does not create a code violation for A36. For QT steels, Clause 7.7 limits heat input and interpass temperature to protect the tempered microstructure.
GMAW on A36 falls under Category B. For material 3/4 inch and under, the minimum is 32°F. Over 3/4 through 1-1/2 inch, the minimum is 50°F. Over 1-1/2 through 2-1/2 inch, it rises to 150°F. Over 2-1/2 inch, the minimum is 225°F. These same values apply to FCAW, SAW, and low-hydrogen SMAW.
Clause 5.7.2 requires using the highest minimum preheat from either base metal. If you weld A36 (Category B) to A572 Grade 65 (Category C), you use Category C preheat values for the joint. The thicker member at the point of welding determines which thickness band applies.